1. Field of Invention
This invention relates to optical transmission systems and methods.
2. Description of Related Art
The performance of optical communication systems using optical fiber is constrained by noise accumulated in passing through optical amplifiers in the communication system, and by non-linear effects on transmitted signals that are launched with a high signal power. High initial signal power is desirable to overcome accumulated noise, but high initial signal power also tends to increase the effects of fiber non-linearity on the signal. Thus, a balance between signal power, noise and non-linear effects on the signal is required.
The effects of fiber non-linearity can be controlled to some extent by proper management of chromatic dispersion in the transmission system, such as that described in U.S. Pat. No. 5,559,920 to Chraplyvy et al. Typical dispersion management techniques attempt to keep the overall dispersion near zero while keeping the local dispersion away from zero to control non-linear effects. However, the accumulated dispersion at any point is restricted to levels such that the data pulses never become severely distorted.
Other techniques to overcome deleterious non-linear effects have also been demonstrated. Soliton transmission, for example, balances the combined effects of nonlinearity and dispersion to maintain optical pulse integrity. The widening effect of dispersion is balanced by the peaking effect of the non-linearity, so that the optical pulses propagate nearly unchanged. A similar technique, referred to as xe2x80x9cchirped RZxe2x80x9d transmission, adds small amounts of phase modulation to the pulses, again with the intention of maintaining the pulse shape as undisturbed as possible.
The invention provides an optical communication system and method that reduces the effect of non-linearities on a transmitted signal. The invention works on a principle opposite to those described above, by rapidly dispersing the transmitted optical pulses. In one aspect of the invention, transmitted signals and/or the transmission system are arranged so that the transmitted signals disperse rapidly in the transmission medium. For example, a transmitted signal can be generated to have a relatively high number of constituent frequencies, i.e., a wide frequency bandwidth. We define a xe2x80x9cdispersion-enhancedxe2x80x9d signal to be one which occupies a frequency bandwidth wider than that required for a given bitrate, e.g., 2*bitrate for a conventional non-return to zero (NRZ) signal, for the express purpose of reducing the dispersion length. When this dispersion-enhanced signal is launched into the transmission medium, e.g., a conventional optical fiber, chromatic dispersion in the transmission medium causes the different frequencies to propagate at different speeds, thereby dispersing the signal. At a bitrate of 40 gigabits per second (40 Gb/s) where the bit period is 25 picoseconds (ps), a dispersion-enhanced signal having a relatively wide frequency bandwidth can be created, for example, by constructing the signal from very narrow pulses, e.g., 3-ps pulses. The dispersion-enhanced signal can also be created by xe2x80x9cchirpingxe2x80x9d the conventional signal (adding extra frequency spectrum through additional phase or frequency modulation), or by using a light source that has a broadened linewidth, e.g., 40-200 GHz or broader. The transmission system can also include components that disperse a transmitted signal rapidly. For example, an optical fiber link in the system can have an amount of chromatic dispersion that causes a dispersion-enhanced signal or other standard signal to disperse rapidly. Other components of the system can receive standard communication signals, and alter the signals so that the signal is dispersion-enhanced, e.g., has a wide frequency bandwidth.
Although the optical transmission system has one or more portions that cause a signal to rapidly disperse, the system can also incorporate dispersion compensation to reverse the effects of dispersion on a signal. For example, an optical transmission fiber having a given chromatic dispersion that is used to transmit a signal can provide the signal to a dispersion compensating fiber that has a chromatic dispersion opposite the transmission fiber. Thus, while the transmission fiber causes the signal to be broadly dispersed, the compensating fiber can cause the signal to be drawn back together into a more well-defined, non-dispersed signal so that it can be properly received.
The optical transmission system and method can also mitigate the effects of residual dispersion and dispersion slope in the transmission system, for example, by performing optical filtering at receiver portions of the system. Dispersion in the system causes different wavelengths of light to experience different propagation speeds. Dispersion slope in the system causes different wavelengths of light to experience different amounts of dispersion compared to other wavelengths. By appropriate narrow filtering of the dispersion-compensated signals, dispersion and dispersion slope in the transmission system need not be perfectly compensated.
Proper optical filtering at a receiver portion can also allow close channel spacing, e.g., for wavelength-division multiplexing, and even allow some overlap between adjacent channels. That is, even though the frequency content of adjacent channels may overlap somewhat, proper optical filtering can select the information from a desired channel without a significant effect on signal quality.
These and other aspects of the invention will be apparent and/or obvious from the following detailed description.